M1 is Ti, Zr or Hf, n is 2, R1 is hydrogen or an organic radical which has from 1 to 20 carbon atoms and is unbranched in the ?? position, in particular an organic radical which has from 1 to 20 carbon atoms and is unbranched in the ?? position, R3 is a substituted or unsubstituted C6-C40-aryl radical and R5 is an organic radical which has from 3 to 20 carbon atoms and is branched in the ?? positi